Solid oxide fuel cells (SOFC) have an electrolyte comprising an oxide ion conductor such as stabilized zirconia, for example. In the solid oxide fuel cells, an electrolyte electrode assembly (membrane electrode assembly), which is also referred to as MEA hereinafter, including an anode and a cathode that are disposed one on each side of the electrolyte is sandwiched by separators (bipolar plates).
The electrolyte electrode assembly comprises a stack body of an electrolyte as a dense layer and electrode layers as porous layers. To produce the electrolyte electrode assembly, a plurality of firing steps are performed to form an electrolyte and electrode layers successively.
One way of making the overall process of producing an electrolyte electrode assembly efficient would be to fire an electrolyte and electrode layers integrally together. Since the dense layer and the porous layers are made of different materials, it is necessary to accurately inspect whether the fired stack body (electrolyte electrode assembly) is acceptable or not.
Japanese laid-open patent publication No. 2002-090346, for example, discloses a method of inspecting a ceramics sheet for through pores. According to the disclosed method, an insulative ceramics sheet is sandwiched between two parallel electrode plates, and a discharged current generated when a high DC voltage is applied between the electrode plates is detected to inspect whether there are through pores having a minimum length of 2 Å in the insulative ceramics sheet or not.
Japanese laid-open patent publication No. 2004-327129 discloses an apparatus for inspecting a hollow cylindrical solid oxide fuel cell. According to the disclosed apparatus, a cell tube comprising a plurality of unit cells, each comprising an air pole, an electrolyte, and a fuel pole which are disposed on a base tube, connected together by an interconnector, is inspected for performance at a high temperature. The apparatus includes a continuous furnace having a temperature increasing zone, a high temperature holding zone, and a temperature lowering zone that are successively arranged from an inlet side, supports for supporting the upper ends of cell tubes, and a feed means for feeding the supports. A fuel supply unit to the cell tubes and a fuel discharge unit from the cell tubes are kept at normal temperature. At the same time, the cell tubes are successively moved from the temperature increasing zone, the high temperature holding zone, and the temperature lowering zone. In the high temperature holding zone, the cell tubes are inspected for performance and also inspected for strength screening at a high temperature based on stresses applied to lower portions of the cell tubes.
Japanese laid-open patent publication No. 2005-108801 discloses an apparatus for detecting a defect in a stack body of at least a porous layer and a dense layer. The apparatus comprises a heating source or a cooling source and a temperature distribution detector. The heating source or the cooling source is disposed near the porous layers for uniformly heating or cooling inspected regions of the porous layers. The temperature distribution detector is disposed near the dense layer for detecting the distribution of heat transmitted from the porous layers to an inspected region of the dense layer which lies adjacent to the inspected regions of the porous layers in the direction along which the dense layer and the porous layers are stacked.
According to the method disclosed in Japanese laid-open patent publication No. 2002-090346,the high DC voltage has to be applied to all insulative ceramics sheets to inspect them for through pores. Consequently, the inspecting process is complex and time-consuming. In addition, the apparatus requires a high voltage applying device.
Furthermore, the disclosed apparatus is only designed to inspect an insulative ceramics sheet to check if there are through pores in the insulative ceramics sheet or not, and is unable to inspect an insulative ceramics sheet for its structural density, making it difficult to determine whether the current-voltage characteristics of the MEA are acceptable or not. In addition, the high DC voltage applied to the stack body tends to degrade the stack body.
According to the apparatus disclosed in Japanese laid-open patent publication No. 2004-327129,all cell tubes have to be inspected for performance at a high temperature. Therefore, many inspecting steps and a long inspecting time are required, and various inspection facilities including a high temperature furnace, a heater, etc. are necessary. The disclosed apparatus is thus not economical. Furthermore, the cell tubes may possibly be degraded due to temperature changes and oxidation and reduction behaviors during the inspecting process.
According to the apparatus disclosed in Japanese laid-open patent publication No. 2005-108801,all stack bodies are heated or cooled and the distribution of heat transmitted from the porous layers of each of the stack bodies is detected. Therefore, many inspecting steps and a long inspecting time are required, and various inspection facilities including a heater, a Peltier device, an infrared camera, an ultrasonic transducer, an ultrasonic probe, etc. are necessary. The disclosed apparatus is thus not economical. In addition, the disclosed apparatus fails to check the structural density that greatly affects the conductivity of the stack bodies. The stack bodies may possibly be degraded due to temperature changes, vibrations, etc. during the inspecting process.